feat: skip saving head system messages in history

[Soulter]

Modifications / 改动点

• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

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Checklist / 检查清单

• 😊 如果 PR 中有新加入的功能，已经通过 Issue / 邮件等方式和作者讨论过。/ If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
• 👀 我的更改经过了良好的测试，并已在上方提供了“验证步骤”和“运行截图”。/ My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
• 🤓 我确保没有引入新依赖库，或者引入了新依赖库的同时将其添加到了 requirements.txt 和 pyproject.toml 文件相应位置。/ I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
• 😮 我的更改没有引入恶意代码。/ My changes do not introduce malicious code.

Summary by Sourcery

增强内容：

• 修改历史记录保存逻辑，仅忽略最初的系统消息，并将后续的系统消息包含在已存储的历史记录中。

Original summary in English

Summary by Sourcery

Enhancements:

• Change history saving logic to ignore only the initial system message and include later system messages in stored history.

[sourcery-ai]

Hey - 我在这里给出了一些总体反馈：

• 新增的 got_first_system 标志可以正确保留后续的 system 消息，但它的意图不是特别直观——可以考虑用一个更清晰的名字（例如 skipped_initial_system），以及/或者加上一条简短注释，说明为什么只将第一条 system 消息排除在历史记录之外。

给 AI 代理的提示

Please address the comments from this code review:

## Overall Comments
- The new `got_first_system` flag correctly preserves later system messages, but its intent isn’t immediately obvious—consider a slightly clearer name (e.g. `skipped_initial_system`) and/or a brief comment explaining why only the first system message is excluded from history.

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Original comment in English

Hey - I've left some high level feedback:

• The new got_first_system flag correctly preserves later system messages, but its intent isn’t immediately obvious—consider a slightly clearer name (e.g. skipped_initial_system) and/or a brief comment explaining why only the first system message is excluded from history.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The new `got_first_system` flag correctly preserves later system messages, but its intent isn’t immediately obvious—consider a slightly clearer name (e.g. `skipped_initial_system`) and/or a brief comment explaining why only the first system message is excluded from history.

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[Soulter]

已经在 73fca5d 修改为只跳过第一个 system message